It is known that reception of radio-frequency (RF) signals can be improved through a diversity technique using multiple antennas to maximize the signal quality of the received signals. The signals from the different antennas can be combined in a variety of ways, but the simplest is to switch between and among the several antennas in order to identify and select the particular antenna providing the strongest signal. Such switching is typically accomplished using an RF switch, and selection of the particular antenna may be based on such measures of signal quality as, for example, higher received power or lower bit error rate. The best performance in a diversity antenna system is obtained by switching based on bit error rate, which avoids erroneous signal power measurements that can be caused by interfering signals. In order to minimize cost and simplify construction, it is often desirable to use a blind switching system based on a threshold bit error rate value and measuring the bit error rate only for the antenna that is currently selected. If the bit error rate exceeds the pre-established threshold value, then the diversity switch selects a different antenna. This system is based on the assumption that the signal from the different antenna cannot be worse than the signal from the currently-selected antenna because the latter is already performing at or below the threshold value and therefore providing unacceptable signal quality. If the two antennas are statistically independent, then there is a good chance that the different antenna will provide a stronger signal. This is known as diversity gain. The different antenna could provide worse reception, but all signals below the threshold value are considered to be equally unacceptable, particularly for digital systems. By optimizing the pre-established threshold value, such a system can perform nearly as well as a system that measures bit error rates for both antennas simultaneously, while eliminating redundant hardware.
Referring to FIGS. 1 and 2, prior art diversity systems 20,30 typically comprise a radio receiver 22,32; two or more antennas located either in separate antenna modules 24a,24b, as shown in FIG. 1, or in the same antenna module 34, as shown in FIG. 2; and separate cables extending between each antenna and the radio receiver 26a,26b,36a,36b. Optimum performance is achieved when the antennas have complimentary radiation patterns 28a,28b,38a,38b. This be achieved either by physically positioning the antennas such that their radiation patterns 28a,28b and maximum gain are in opposite directions, such as at and toward the front and rear of a vehicle, as shown in FIG. 1, or by co-locating and combining the antennas to produce the complimentary radiation patterns 38a,38b, as shown in FIG. 2. The latter implementation can be accomplished by adding the signals from antennas with different phases, such as, for example, by combining two antennas that are separated by one-half wavelength with a phase difference of either 0° or 180° to produce sum and difference patterns which are complimentary.
Choosing between a diversity antenna system architecture with multiple antenna modules, each having a single antenna, and a system architecture with a single antenna module having multiple antennas often depends on such practical considerations and constraints as hardware and installation costs, vehicle design, and antenna placement, and may not depend on maximizing performance. For example, diversity is often used to achieve only nominal performance which is no better than that provided by a single omnidirectional antenna, but using two antennas that are hidden to preserve the integrity of the vehicle's style. Each of these hidden antennas can have lower performance requirements than the single omnidirectional antenna, and can therefore be integrated more easily into a vehicle or hidden within the vehicle structure.
In each of the two aforementioned diversity antenna system architectures, switching between and among the antennas is performed at the radio receiver 22,32, which requires the separate cables 26a,26b,36a,36b extending between the receiver 22,32 and each of the antennas, resulting in both a multiplicity of cables and a corresponding number of RF antenna connectors at the receiver 22,32. The cost of RF coaxial cable is calculated based on length, and, particularly in a large-scale structure such as a vehicle, the cost of this multiplicity of cables can approach the cost of the antenna modules. Furthermore, packaging and routing the many cables can be time-consuming and labor-intensive.
Prior art diversity control systems are possible that use a lesser number of cables, but these systems rely on undesirably complex, large, or costly technology, such as filtering mechanisms, and complex control signals to switch between antennas.
Due to these and other problems and limitations in the prior art, an improved control system and method for controlling switching in diversity antenna systems is needed.